We use dovetails to secure (almost) all of our joists and purlins. The dovetail pocket is cut about 1/4″ bigger than the dovetail itself.
This allows the dovetail to slip in easily when you are setting the joists or purlins and then move on to raising the rest of the frame and come back to tighten the joint later. The geometry of the joint will ensure that it will not slip out while you continue to set other pieces. To tighten the joint the two rafters, or girts, depending on if it is joists or purlins are pulled tight with a come along and nylon straps to prevent any damage to the timbers and then hardwood (usually Maple) shims are driven in between the dovetail and the pocket pulling the joint in tight.
You must be certain that you are ready to tighten that particular joint though as once the shims are put in place it is very hard if not impossible to get them out.